Primary malignant brain tumors have an extremely poor prignosis. These malignancies are athe most common solid tumor in children and the third leading cause of cancer deaths in persons 15 to 34 years of age. Moreover, the incidence of brain tumors is increasing in those older than 65. CENUs (chloroethylnitrosoureas) are alkylating agents which, when used in single agent or combination chemotherapy along with surgery and radiation, are one of the most effective antitumor drugs for treatment of adult and pediatric brain tumors. However, intrinsic and acquired resistance to CENUs limits their usefulness. The broad, long term objective of this proposal is to ascertain the mechanisms of brain tumor resistance to chemotherapeutic alkylating agents (CENUs and methylating agents) and to identify strategies to combat resistance. We have shown that resistance of 14 human brain tumor cell lines to CENUs and methylating agents is multifactorial, and is based on a mechanism(s) in addition to the DNA repair protein O6. methylguanine-DNA methyltransferase (MGMT). Our hypothesis is that base excision repair of cytotoxic DNA lesions, e.g., abasic sites, is another, as yet undocumented resistance mechanism. To address this hypothesis, we will use antisense oligonucleotides to suppress key enzymes in base excision repair in a previously analyzed panel of brain tumor cell lines where we have recently quantiated alkylating agent sensitivity nd the contribution of MGMT to resistance. The base excision repair enzymes we will target. separately and in combination, are apurinic/apyrimidinic endonuclease and DNA polymerase beta. We will quantitate the effect of antisense oligonucleotides on mRNA and enzyme levels, and on CENU and methylating agent cytotoxicity in the absence and presence of a substrate analogue inhibitor of MGMT. In related work, we will quantitate apurinic/apyrinidinic endonuclease and DNA polymerase beta, together with MGMT, in newly diagnosed and recurrent brain tumors, and assess the relationship of enzyme levels to response to CENU therapy. The tissue studies, together with the in vitro work, will allow us to identify potentially effective alkylating agent/inhibitor therapies. If our hypothesis is correct, inhibitors of base excision repair enzymes could eventually be tested clinically, either with or without an inhibitor of MGMT (e.g., O6-benzylguanine). It is a logical expectation that a combination of differentially targeted inhibitors might effectively potentiate CENU and methylating agent chemotherapy for adult and pediatric brain tumors.